1. Field of the Invention
The present invention relates generally to the field of hazardous waste containment and disposal and, particularly to a method of long term storage which allows, when necessary, easy retrieval of the stored waste material.
2. Description of the Problem
Hazardous wastes include a variety of toxic and radioactive materials which have the capacity to cause considerable harm if released directly into the environment. The various types of hazardous waste materials and methods for storing or otherwise disposing of them ar defined generally in regulations and other documents issued either by the various state or federal agencies concerned with environmental safety. Federal agencies involved in this activity include the Environmental Protection Agency (EPA), the Occupational Health and Safety Agency (OSHA) and, where radioactive materials are involved, the Nuclear Regulatory Commission (NRC), a part of the Department of Energy (DOE).
With respect to radioactive wastes, NRC has established a number of classes with respect to the containment and disposal requirements therefor as follows:
Class A: mildly hazardous "segregated" wastes for which there are no stability requirements but which must be disposed of in a segregated manner from other waste materials. Class A wastes are defined in terms of maximum allowable concentrations of certain isotopes and are subject to certain minimum requirements regarding waste form and packaging for safe handling. Typical materials falling in this category are gloves and other protective clothing worn by workers in nuclear facilities, contaminated animal carcasses and laboratory equipment, all of which are generally contaminated with an average level of radioactivity of about 0.1 curies per cubic foot. It has been reported that in 1988 alone over two million cubic feet of such waste materials were produced.
Class B: hazardous "stabilized" waste having higher levels of radioactivity and a need to be placed in a stable form and disposed of in a segregated manner from unstable waste materials. Class B wastes include materials such as piping, pumps and valves from nuclear reactor facilities, radium contaminated waste tailings from uranium mines and similar materials. Materials in Class B are also defined in terms of maximum allowable concentrations of certain isotopes and requirements for a stable waste form as well as minimum handling requirements.
Class C: extremely hazardous "Intruder protected" waste materials which need to be placed in a stable form, disposed of in a segregated manner from nonstabilized waste forms and then disposed of in such a manner that a barrier is provided against potential inadvertent intrusion and contact after institutional controls have lapsed. Class C waste materials are also defined in terms of maximum allowable concentrations of certain isotopes and typically comprise items such as spent fuel rods, safety and control rods, core barrels, items directly exposed to primary coolant, and other wastes contaminated with radium, thorium, uranium or one or more transuranic elements. In the case of spent fuel rods, radiation levels in excess of 200,000 curies per cubic foot may be encountered. Because of the great volume and extremely long half lives of some of these materials, the mode of disposal most commonly proposed involves the "immobilizing" of the waste materials and their deep burial for very long periods of time. The time period of such burial depends upon the amount of radiation involved and the isotopic half lives of the stored material with a period of 10,000 years presently being required for the most highly radioactive materials, such as spent fuel rods and similar materials. As a practical matter, this means that such containment and storage would be considered as being permanent.
To achieve this degree of storage, a variety of approaches have been proposed. One method of immobilizing these waste materials comprises grinding or shredding them, along with the containers used to hold them, and then incorporating the shredded materials into a solid matrix such as concrete for burial underground. While encapsulation of the containers in concrete would appear to provide the necessary level of long term stability, a concrete sheath can fail from earth movement resulting from earthquakes, earth subsidence and the like. Such earth movements frequently can fracture even the most solid containment matrixes over a long period of time. Furthermore, fracture can also occur as a result of the internal pressure developed by the expansion and dehydration of the stored material as it is heated by radioactive decay, and by the release of various gases such as steam, radon and helium by the decay process.
It has also been suggested in U.S. Pat. No. 4,701,280 that the most highly radioactive materials be permanently stored at great depth in specially designed chambers hollowed out under primary rock. However, primary rock contains cracks and cavities and often also aquifers. The rock can also be subjected to deformation, for example, from earthquakes and other seismic activity, earth subsidence and the like. Consequently, there is at least some degree of risk that such deformations, combined with any corrosion resulting from contact of the radioactive materials storage containers with any ground water seeping into the chambers holding such waste, will cause the waste containers to rupture and allow the released materials to spread without control.
Other methods of hazardous waste containment are described in U.S. Pat. Nos. 4,784,802 and 4,844,840 and include the placement of arrays of waste-containing modules in an earthen trench or depression covered by a non-rigid cap or earthen cover. Such storage configurations are sensitive to earthquakes and require special structures for handling the seepage of ground water.
Most recently, the DOE has proposed storing large quantities of high level radioactive materials in deep chambers hollowed out in salt beds underlying large areas of the western United States. Such beds are thought to be relatively free of long term problems resulting from siemic activity and ground water seepage. To determine the long term viability of this approach to storing high level wastes, work has been started to design and construct a waste isolation pilot plant (WIPP) which will study and define the problems involved in storing up to about 6,000,000 cubic feet of radioactive waste materials for extremely long periods of time. It is contemplated that this facility will, within a period of about 50 years, demonstrate the utility of one or more methods for achieving compliance with the presently allowable radioactive isotope release limits over a 10,000 year period.
At the present time, however, none of the previously referred to approaches to waste containment appears useful in the WIPP facility. This is due to the natural plastic flow of salt within the salt beds in which the waste containing chambers will be formed and the need to provide for periodic inspection of the waste holding containers over the 50 year test period to verify the continued safety and utility of the waste containment structures. The natural flow of salt within the salt bed will result in extremely high compressive forces on the waste holding containers, much greater than those contemplated in the previously proposed approaches to waste containment. Furthermore, it is contemplated that over a period of about 50 years, the salt flow will progress to the point where it will compress the entombed containers sufficiently to lock the containers in place and cause many of them to rupture. As a result of such compression and locking, any removal, inspection or repackaging of leaking containers will become, as a practical matter, a physical impossibility. What is needed is a storage method that will distribute the compressive loads of the migrating salt and allow for retrieval, inspection and, if necessary repackaging of the waste holding containers over a time frame of sufficient duration to assess the long term performance of the salt bed depository. The present invention satisfies that need.